Process of preparing boron carbides



Jn- 14, 1941- w. KAUFMANN ETAL 2,228,923

PROCESS OF PREPARING BORON CARBIDES Filed July 25, 1958 2 sheets-sheet 1Jan. 14, 1941. W KAUFMANN HAL 2,222,923

PROCESS OF PREPARING BORON CARBIDLS Filed July 25, 1958 2 Sheets-Sheet 2INVENTORS 2 ATTOFIN EY Patented Jan. 14, 1941 UNITED STATES PROCESS OFPREPARING BORON CARBIDES Waldemar Kaufmann and Alwin Krauss,Frankfort-on-the-Main-Griesheim, Germany, assignors, by mesneassignments, to Walther H. Duisberg, New York, N. Y.

Application July 23, 1938, Serial No. 220,994 In Germany January 30,1936 3 claim.

This invention relates to a process of preparing boron carbide byheating boric acid and carbon. 'I'his application is acontinuation-impart of U. S. patent application Serial No. 122,270,filed 5 January 25, 1937, in the name of Waldemar Kaufmann and AlwinKrauss.

In that specification there is described a proc.. ess of preparing boroncarbide which comprises causing a mixture of commercial hydrous boricacid (HaBOa) and carbon to react by the action of an electric arc.

A known process of preparing boron carbide consists in heating a mixtureof dehydrated boric acid (B203) and petroleum coke in an electricresistance furnace. Mixtures with hydrous boric acid cannot be used forthis purpose because, owing to the str'ong formation of foam attendingthe dehydration of the boric acid, the material lying round thecentrally situated heating resistance is removed from its position;moreover the steam formed in the interior which has to flow through thewhole material would react undesirably with the carbon. The dehydrationof commercial hydrous boric acid (H3BO3) on a large scale is diflicultbecause ceramic material as well as metals are attacked by the fusedboric acid.

New we have found that boron carbides may directly be prepared frommixtures of commercial hydrous boric acid and carbon such as petroleumcoke or the like by heating the mixture not in an electric resistancefurnace but by means of an electric arc. In preparing the mix- I ture ofboric acid and carbon the proportion of 35 boron to carbon in thefinished product may be varied within large limits. Especially B4G andBeC may readily be obtained in a good yield.

For instance on a relatively thin layer of a mixture of commercialhydrous boric acid and petroleum coke in any suitable container of heatresisting material an electric arc is generated between at least twocarbon electrodes introduced from above. In this case the expressionrelatively thin relates to the thickness of the layer in comparison withthe dimensions of the container and the electrodes. Depending upon thedimensions of the container the layers of the parent mixture introducedhave a thickness of some centimeters, for instance about 3 to 25centimeters. Further quantities of the mixture are then carefully addedin the form of further layers lying one above the other until theelectric arc is covered. A layer of boron carbide fused on the surfaceis soon formed round the arc and especially below it, while watervigorously evaporates. The steam may readily escape through therelatively thin layer of the reacting mass covering the arc and maylikewise flow off through the hollow space formed round the electricarc. Fresh material may slowly be added continuously or in the form offurther thin layers carefully placed one above the other as thedehydration of the layers around the hollow space and especially ofthose covering it from above and the formation of finished boron carbideare proceeding by the heat of the electric arc. The radiation of the arcdirectly transforms the mixture surrounding it into boron carbide, withevolution of carbon monoxide. The boron carbide rst accumulates in theform of a liquid below the electrodes. Thereby and by pushing forwardthe dehydrated portions of the reacting material the quantity of thefused boron carbide may continuously be increased so that, by raisingthe electrodes and continuously charg- 5 ing fresh starting mixture ablock of boron carbide is obtained which solidies from below andincreases upwards. It is immaterial whether small quantities ofincompletely dehydrated mixture come into the molten mass because theWater contained therein instantly evaporates and can readily escapealong the electrodes together with the carbon monoxide formed.

Of course, the process may be varied. For instance a three-phase processwith three electrodes or a single-phase process with one electrode andthe molten mass as second electrode may be applied. In the latter casefirst only one electrode may be placed in the mixture and a secondcurrent-carrying agent, for instance a carbon plate, is supplying thecurrent from below.V After a part of the boron carbide has been fusedthis molten mass serves as the second electrode.

It is possible, for instance, to introduce a current-carrying agent intothe mixture from below through an opening in the wall of the containerand to place above the layers one or more electrodes. By the action ofthe electric arc boron carbide is first formed between the variouscurrent conductors and this boron carbide, in the liquid as well as inthe solidied state, may in the further course of the reaction act asconductor to the electric arc.

In the following two modes of operating the process which have proved toinvolve a special industrial value are described.

The accompanying drawings diagrammatically illustrate the preparation ofboron carbide according to the invention, Figs. l to 5 being 50 verticalcross-sections through the electric arc.

Figs. l, 2 and 3 are cross-sections through an arrangement of twoelectrodes introduced from above,

Figs, 4 and 5 are cross-sections through a de- 55 vice with oneelectrode introduced from above, the second current conductor beingplaced below.

According to Fig. l a layer 2 of the starting material of a thickness ofabout l5 centimeters is introduced intera container 5 of heat resisting60 material, for instance of metal or a ceramic' mass. A suitablemixture of starting material consists of 100 parts by weight ofcommercial hydrous boric acid (HsBOa) and 30 parts by weight ofpetroleum coke. This mixture, when used in the process described in thefollowing passage, yields a product with a content of boron of '78 percent. The invention, however, is not limited to this proportion, butother mixing proportions may likewise be applied. By the increase of thecontent of boric acid in the mixture necessary for the preparation ofproducts especially rich in boron the losses by evaporation of the boricacid also increase so much that it is advisable to recover thesequantities with the aid of suitable separators.

Directly above the layer 2 of the starting material an electric arc 3 isgenerated in the usual manner between two electrodes l introduced fromabove in an inclined position with a tension of to 100 volts. For thispurpose the electrodes I are brought in contact for a moment and thenseparated from each other.

As it may become necessary to generate the electric arc 3 also duringthe reaction the electrodes l must be supported in such a manner thatthey may readily be moved towards each other and again separated fromeach other. It is, however, also necessary to arrange the electrodes insuch a manner that they may readily be lifted and lowered.

By the action of the electric arc 3 the surface of the mixture 2situated below begins to melt, while vigorously forming steam. As soonas the formation of steam ceases the electric arc 3 is carefully coveredwith a further layer of the mixture. If the arc is extinguished by thislayer it must again be generated in the above described manner. Furthersteam is formed and the material agglomerates and in a short time ahollow space 4 is formed around the electric arc 3 which hollow spaceincreases in proportion with the reaction of the surrounding mass causedby the heat. The boron carbide formed melts and accumulates in the formof a thin layer 6 `below the electric arc 3. The electrodes l andtherewith the arc 3 are then somewhat lifted and the layer covering thearc is increased by the addition of further mixture. This new layer issoon dehydrated by means of the hot material situated below and `by theflame -of carbon monoxide. The steam escapes upwards into the airwithout having an opportunity of reacting with the boron carbide formed.As by the radiation of the electric arc only a thin layer of the boroncarbide is maintained in the molten state the mass solidies below thislayer so as to form a solid block 5 of boron carbide. By lifting theelectric arc and adding new material new layers are permanently formedand the block 5 increases thereby. The block is surrounded by a zone 8.Near the block of boron carbide it consists of non-molten boron carbide,towards the outside it becomes more and more spoiled with the crudematerials. The outer layer 2 consists of more or less stronglydehydrated pure starting mixture. The hard block of boron carbide mayreadily be separated from the outer layer 8 with which it is onlyloosely associated; this is done, for instance, with a brush of steel.

In the case of large containers with carbon Velectrodes of, forinstance, a diameter of 25 centimeters and a length of several metersthe high weight of these electrodes, when they are arranged in aninclined manner, necessitates complicated regulating devices. Theirregular burning off of the thick coals also aggrava-tes the adjustmentof the desired distance. Therefore it is necessary to apply anarrangement for regulating the distance of the electrodes determiningthe amperage, for instance an arrangement according to Figs. 4 and 5 ofthe drawings.

A circular stick of coal l movably suspended, for instance, by a rope orcable and a coal-plate l horizontally placed thereunder serve aselectrodes. The circular stick of coal I is set on the coal-plate and amixture of boric acid and petroleum coke is piled around it. After thecurrent has been switched in the electric arc is generated by liftingthe round coal. By the radiation of the electric arc a hollow space 4 issoon formed in in the surrounding mixture. The solid boron carbide 5formed in the same man-ner as described above settles on the coal-plate'I in the form of a thin, continuously increasing layer. As the boroncarbide conducts electricity the current is not interrupted thereby. Ashort time after the beginning of the reaction the electric arc,therefore, burns between the circular stick of coall and the block ofboron carbide 5 or the molten Vpart 6 covering its surface. Theoperation ,is performed. as described above. A difference only consistsin the following: the dehydrated material introduced cannot as easily asin the foregoing case reach the reaction Zone by itself so that fromtime to time the material round Ythe coal must be pushed down,preferably with a wooden stick. Small quantities of incompletelydehydrated boric acid which at this moment fall into the reaction zonedo not disturb as they are at once dehydrated.

There may, of course, also be worked with a three-phase current eitherby bringing two sticks of coal l from above towards one coal-plate 'lwhich may, for instance, form the bottom of the container, or by usingthree sticks of coal l and applying the ycoal-plate 'l as zero in starconnection.

We claim:

1. A process of preparing boron carbide from a mixture of commercialhydrous boric acid and carbon which comprises introducing into acontainer of heat-resisting material a small amount of a mixture forminga relatively thin layer on thebottom of said container, generating anelectric arc on said layer whereby the mixture begins to -m'elt formingsteam, adding further, thin layers lying one above the other around theelectric arc, as soon as the formation of steam ceases from each layer,heating each layer in the same manner as said first layer, continuing toadd said. thin layers until a hollow space is formed around the electricarc substantially consisting of molten boron carbide, and then raisingthe electric arc and repeating the above procedure until a solid blockof boron carbide is formed 'below the thin layer of molten boron carbidesurrounding the arc.

2. A process in accordance with` claim 1 in which each charge forms a'layer of a thickness of about -15 centimeters.

3. A process in accordance with claim l in which each charge is amixture of parts by weight of commercial yhydrous boric acid and. of 30parts by weight of petroleum coke, and in which each layer has athickness of about l5 centimeters.

WALDEMAR KAUFMANN. ALWIN KRAUSS.

